Questions tagged [gis-principle]

Use for questions which involve a GIS principle or theory and are software agnostic

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5 votes
3 answers
168 views

Segmentation of a network at branches from one complex boundary polygon

If I have the polygon defining boundaries of a river network, is there any way to directly segment the tributaries out using geospatial data analysis? To elaborate, I am looking for a workflow more ...
7 votes
2 answers
9k views

Difference between Geodetic Distance and Great Circle Distance?

According to Wikipedia: The great-circle distance or orthodromic distance is the shortest distance between two points on the surface of a sphere, measured along the surface of the sphere (as ...
0 votes
1 answer
200 views

Choosing projection system to use when georeferencing PDF maps

I need to georeference a PDF map and digitize the features on the map into a shapefile using WGS84. Is it best to georeference the PDF map and digitize the associated features in a local projection ...
605 votes
7 answers
571k views

Measuring accuracy of latitude and longitude

I have latitude and longitude as 19.0649070739746 and 73.1308670043945 respectively. In this case, both coordinates are 13 decimal places long, but sometimes I also get coordinates that are 6 decimal ...
1 vote
0 answers
20 views

How to use Lagrange interpolation method for station lies between a grid?

I have rainfall data for a catchment from 1990-2020 in grid format. But my station of interests lies in the grid, how to apply the interpolation to that station. I'm not sure if I'm proceeding ...
1 vote
1 answer
286 views

Latitude represented as a Matrix

I have some NetCDF data where I have a few variables lat : dimensions - (x,y) lon : dimensions - (x,y) x : dimensions - (x,) y : dimensions - (y,) temp : dimensions - (year, y, x) I am struggling a ...
0 votes
0 answers
58 views

Regarding Geographically Weighted Regression (GWR) for non-linear problems

I am new to this topic. I am referring to this Geographically Weighted Regression tutorial. From my understanding, GWR does a weighted regression to model the dependencies between exploratory and ...
143 votes
14 answers
75k views

What are Definition, Algorithms and Practical Solutions for Concave Hull?

Convex Hull A convex hull of a shape is defined as: In mathematics, the convex hull or convex envelope for a set of points X in a real vector space V is the minimal convex set containing X (Wikipedia)...
0 votes
1 answer
139 views

Reason for existence of single parts vs. multipart layers

Vector layers can consist of single part or multipart layers. What is the reason for creating separate formats for that? It is perfectly possible to create a multipart layer and still create just one ...
0 votes
2 answers
84 views

What are the QGIS words for my 4 types of data?

My project has 4 types of data: a raster satellite view (Google Maps), to be the base layer and geolocated by three points (CSV) that apply to visible points on the raster map; individual points: ...
-1 votes
2 answers
98 views

Using GIS to determine which properties have lake view

I'm a real estate agent in the Finger Lakes region of Central New York. Sometimes my clients want a house with a lake view. I am considering commissioning someone to build a map of all the nearby ...
9 votes
1 answer
3k views

MultiSurface Geometry type in QGIS: difference to MultiPolyon

Adding a new temporary scratch layer in QGIS, selecting the Geometry type, there are different options, between else MultiSurface. In the documentation, I could not find any information about this ...
16 votes
4 answers
20k views

Meaning of "Elevation above surface of ellipsoid"?

I'm using implementation of Vincenty's algorithms, and I have their class called "position" that requires lat, lon, and elevation defined by: "Elevation above the surface of the ellipsoid". What is ...
7 votes
2 answers
2k views

Higher resolution of country borders

I have been looking and looking for a long while, but I was unable to find any freely available datasets for country borders with resolution higher than 1:1000000. I found this strange as I thought ...
1 vote
0 answers
120 views

What does unit mean in spatial reference system?

Each spatial reference system contains a definition of a unit. The rectangular coordinate systems mainly use meters, the spherical ones use degrees. You can see the unit of a srs for example here: ...
14 votes
2 answers
3k views

What is smallest contour interval possible when derived from DTM?

I am converting a 2m DTM to contours, and would like to have the smallest interval possible. Is there a theoretical limit, or even a rule of thumb for defining contour intervals based on the ...
2 votes
2 answers
1k views

What would a simple hillshade algorithm be?

I'm curious how GIS systems actually generate a hill-shade from input data like a raster DEM layer. I'd like to implement my own in software (mostly for fun/learning) but I want to start with a ...
3 votes
1 answer
98 views

Does intesect result change with CRS choice?

Question Before (some) spatial analysis, we need to make sure the coordinate reference systems of the layers are the same. I am doing intersect with two layers. I did that under a coordinate before. ...
34 votes
4 answers
8k views

How to represent trend over time?

I have a time series of rasters where the cells of each raster represents some value at a certain time. I would like to generate a map that highlights the magnitude of positive or negative trends in ...
5 votes
1 answer
227 views

Least cost path: what does least cost value represent, based on slope raster layer

Context and question I have a raster layer with slopes in percent and used the least cost path plugin in QGIS to calculate the least cost path from start to end. I try now to understand what the ...
10 votes
3 answers
16k views

Calculating bounding box size?

What formula could I use to calculate the size in Kilometres of a bounding box based on a given South-West latitude/longitude and a North-East latitude/longitude points? The bounding box format is ...
3 votes
1 answer
297 views

What value does slope represent, how is it calculated from neighboring cells of a raster DEM?

In QGIS, I have a Digital Terrain Model. Using Slope (Menu Raster > Analysis > Slope..., in fact using GDAL Slope algorithm), I get an output raster, either in percents or degrees, the same ...
2 votes
1 answer
292 views

Applying a two step floating catchment with boundary limitations

The literature states that gravity models and the two step floating catchment area (which in itself is a version of the gravity model) method for analysing the accessibility of healthcare services by ...
-1 votes
1 answer
113 views

How do you interpret a DEM? [closed]

I have been asking myself a question for some time. How do GIS experts interpret a DEM (1) ? In my opinion, there are two ways of interpreting the value of a DEM pixel : Consider that the pixel value ...
2 votes
3 answers
2k views

Digitizing Map To Scale?

Is it perfectly acceptable to produce a map at a scale of 1:24,000 from a database that was digitized from a 1:100,000 scale source map?
18 votes
4 answers
2k views

Using GIS tools for non-geographical problems

Many other fields have visual/"spatial" problems that can potentially be solved with GIS tools. I know I've used GIS tools for the odd non-geographical task and was wondering how much this happens and ...
1 vote
1 answer
44 views

Something like map algebra or cell statistics for vector format

I work in environmental consultancy and I have recently been tasked with generating the 'Environmental Zoning' for a road project. I use GIS in my everyday work but nothing too advanced. I'm not sure ...
1 vote
1 answer
315 views

Writing about GIS methods on the example of QGIS

I am preparing a thesis that relies heavily on GIS, specifically using the QGIS platform to calculate area extents and similar types of data. I have only recently learned how to use GIS technology so ...
2 votes
2 answers
909 views

Is there any difference between a stream's Longitudinal Profile and its Elevation Profile?

I have derived elevation profile of a river channel using USGS SRTM 1 ArcSec. I am interested to know whether Elevation Profile and Longitudinal Profile are synonymous?
25 votes
6 answers
10k views

Algorithms for matching segments [closed]

What are the best algorithms to match segments? I'm trying to match corresponding segments from two map sources, one less accurate but with segment names, and one more accurate without segment names. ...
1 vote
1 answer
348 views

How Neighbor Contiguity Algorithm actually works

I want to identify neighbors based on queen or rook contiguities in a programming language that does not have the feature yet. I know that tools like spdep, pysal, geoda, and ArcGIS all implement this,...
-2 votes
2 answers
686 views

Concept behind longitudes

Why longitudes are not parallel and why do they meet at the poles? What problem would have happened if like latitudes they were parallel circles running from west to east? I would request the ...
41 votes
3 answers
78k views

Differences between triangulation and trilateration

Looking around I noticed that many people interchange the terms (triangulation and trilateration) for the same sense. What is the correct sense of Triangulation and what are the differences from ...
7 votes
1 answer
174 views

How can I generate a range of pantones for a map?

When I am given a map (or make one...) I often like to get an idea of the range of colours present. One thing this does is it allows me to quickly visualize contrast (helps build maps for colour blind ...
85 votes
2 answers
492k views

Calculating longitude length in miles

Suppose I have the geographic coordinates of "Saratoga, California, USA" as Latitude: 37°15.8298′ N Longitude: 122° 1.3806′ W I know from here that in the case of latitude 1° ≈ 69 ...
2 votes
1 answer
2k views

Getting geotransform in Mercator given pixel size away from origin

I have a radar image with the following information: The projection is Mercator (Mercator_1SP in GDAL) The projection origin is 0º lat 0º lon At a given geographic coordinates (e.g. lon=10º lat=40º), ...
0 votes
1 answer
129 views

Voronoi and distance to the nearest point

I was convinced that Voronoi polygons were created so that each location in a polygon is closer to the origin point present in that polygon than to any other seeds In Snow's famous case study in Soho, ...
64 votes
7 answers
10k views

What strategies, criteria, or rules to use for selecting coordinate systems?

What strategies, criteria, or rules do you use for selecting coordinate systems for (a) storing, (b) analyzing, and (c) displaying GIS data? (I humbly offer my reply to a related question about ...
21 votes
2 answers
23k views

Understanding terms in Length of Degree formula?

Online calculators such as http://www.csgnetwork.com/degreelenllavcalc.html (view page source) use the below formulas to get meters per degree. I understand in general how distance per degree varies ...
12 votes
2 answers
4k views

Understanding the concept of m value

As far as I understand, the m value "is a measured distance along a route" (similar answers here), thus similar to the mileage of a car driving over a highway. The m value defines the point ...
3 votes
1 answer
137 views

Identify the algorithm (not code) for QGIS native tools

The question For someone with no knowledge in C++ (programming language used to write QGIS code), is there a way to identify based on which algorithms the different QGIS native tools are written? ...
3 votes
1 answer
211 views

Are contiguity matrix and adjacency matrix the same?

I'm am sorry for that beginner question. To start the calculation of a spatial auto-correlation value, the book I'm reading writes that we will starting to construct a contiguity matrix. Looking at ...
3 votes
1 answer
202 views

Naming convention for ways of defining direction

The following two conventions for defining direction are very common, but what are they called? 0 at north, clockwise 0 at positive x-axis, counterclockwise Background: I'm writing a function that ...
5 votes
1 answer
150 views

How is datum used in assigning lat and long to point on Earth's surface?

It's accepted wisdom that geographic coordinate systems require a datum in order to assign lat/long to a point on Earth's surface. However, every point in 3d space already has 3d polar coordinates ...
263 votes
11 answers
153k views

Difference between projection and datum

What is the difference between a projection and a datum?
4 votes
1 answer
373 views

How can geographic coordinate system such as WGS84 draw on screen?

A Geographic Coordinate System represents coordinates on a sphere or ellipsoid. To draw these coordinates on the screen does GIS software apply a default projection method, or does it just consider ...
0 votes
0 answers
567 views

Spatial indexes with RTree for polygons

I am trying to understand about how R-tree works for polygon data. In other words, polygon is a list of vertices (points), however, I am wondering how spatial index can recognize the order of points ...
8 votes
2 answers
4k views

Is the 180 west the same as 180 east?

I had a discussion about this topic with a co-worker. We are working implementing a GPS validation system, and I told him the longitude -180 (or 180W) is not a valid longitude because a valid ...
3 votes
0 answers
657 views

Is there a relationship between driving distance and straight line distance?

For fun, I created an FME workflow that emails me the 10 cheapest fuel princes within 10km of my house. I calculate both the driving distance along roads and the straight line distance. Knowing that ...
2 votes
2 answers
1k views

How to interpret Tobler’s First Law of Geography?

Waldo Tobler's First Law of Geography is an axiom in GIS which states that "everything is related to everything else, but near things are more related than distant things" (Tobler, 1969). ...

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